The present invention relates to a liquid crystal cell, and particularly to a method for controlling a pretilt angle direction in liquid crystal cell in which photoreaction occur in a polymerized layer by ultraviolet irradiation.
Liquid crystals are the liquids consisting of anisotropic molecules. The average direction of long axes of these molecules is referred as the director of LC. The director distribution in a LC bulk is determined by its anchoring on the rigid substrates and characterized by the direction of the axes of easy orientation, corresponding to the minimum of the surface energy of an LC, pretilt angle between the axis of easy orientation and the substrate plane, and tilt angle between the director of LC and the substrate plane.
In order to obtain the uniform brightness and high contrast ratio of a liquid crystal display, the LC molecules must be appropriately aligned after being injected between the substrates of the cell. Not only the value of the director tilt but the direction of this tilt (i.e. direction of the axis of easy orientation) is important for normal operation of LC devices constituting double- and multi-domain structure. Such alignment is achieved by providing an alignment layer on the surface of the substrate. A rubbing process can be used for aligning liquid crystal molecules. In this rubbing process, a polyamide alignment layer is first coated on the substrate and the rubbing is performed mechanically, so that microgrooves are formed on the surface of the alignment layer. The liquid crystal molecules are thus uniformly aligned due to the intermolecular interaction between the polyamide molecules and the liquid crystal molecules.
In the above described rubbing process, however, defects are formed in the microgrooves which cause light scattering and random phase distortion. Also during the rubbing process, dust and electrostatic charges are produced in the alignment layer, so that the substrate is damaged and yield is decreased.
To solve the aforementioned problem, photo-alignment process has been recently introduced. As an example of the photo-alignment method, a method has proposed by KOBAYASHI, etc. (SID 95 DIGEST, p.877) in which the pretilt angle direction is determined by irradiating the UV light twice into an alignment layer consisting of polyvinylcinnamate (PVCN) based polymer, as shown in FIGS. 1A and 1B.
In particular, as shown in FIG. 1A, when the linearly polarized UV light irradiates to alignment layer 15 in the direction perpendicular to the surface of the substrate 16, the alignment layer 15 becomes a photo-polymerized due to cross linking between polymer molecules. The bonding direction of the photo-polymer molecules depends on the polarization direction of the linearly polarized UV light. The liquid crystal is thus aligned according to the bonding direction of the photo-polymer molecules.
Then, the linearly polarized UV light whose polarization direction is perpendicular to the polarization direction of the first UV light is irradiated at an angle .phi. to the alignment layer 15. The pretilt angle of the alignment layer 15 is formed in this step and the magnitude of the pretilt angle varies according to the irradiation angle of the UV light. For example, the pretilt angles are approximately 0.15", 0.26", or 0.30", when the irradiation angles are 30", 40", or 60", respectively.
In KOBAYASHI, however, the method has some drawbacks the thermostability of the tilt angle on the PVCN based materials is poor, the scope of the pretilt angle is small and does not cover the range needed for an applications, only polarized exciting light could be used. Moreover, the method requires rather complicated geometry of the irradiation and suitable for the only materials revealing the light-induced easy axis direction perpendicular to the polarization of the exciting light.